Filtration clogging resistance

Nonwoven filter media are mostly used for filter medium filtration with pore clogging. Because of the relatively low cost of this medium, it is often replaced after pore clogging. In some cases, nonwoven media are used for cake filtration. In this case, cake removal is so difficult that it must be removed altogether from the filter medium. Nonwoven filter media can be prepared so that pore sizes decrease in the direction from the surface of the filter media contacting suspension to the surface contacting the supporting device. This decreases the hydraulic resistance of... 

Care must be taken with both sintered glass and porous base crucibles to avoid attempting the filtration of materials that may clog the filter plate. A new crucible should be washed with concentrated hydrochloric acid and then with distilled water. The crucibles are chemically inert and are resistant to all solutions which do not attack silica they are attacked by hydrofluoric acid, fluorides, and strongly alkaline solutions. 

In the second type of filtration, depth or deep-bed filtration, the particles penetrate into the pores of the filter medium, where impacts between the particles and the surface of the medium are largely responsible for their removal and retention. This configuration is commonly used for the removal of fine particles from very dilute suspensions, where the recovery of the particles is not of primary importance. Typical examples here include air and water filtration. The filter bed gradually becomes clogged with particles, and its resistance to flow eventually reaches an unacceptably high level. For continued operation, it is therefore necessary to remove the accumulated solids, and it is important that this can be readily achieved. For this reason, the filter commonly consists of a bed of particulate solids, such as sand, which can be cleaned by back-flushing, often accompanied by... 

Colloidal filtration is selected as the dip-coating mechanism for the first trials in the development path. This means that cake filtration should occur when the suspension comes into contact with the substrate. So the particle size in the suspension should not be much smaller than 1 pm (approximately 4 times less than the mean pore size in the substrate) otherwise too much penetration and clogging of the substrate occurs prior to cake build-up. This would give rise to an extra high interfacial flow resistance during application of the MF membrane. 

Capillaries with an outer diameter of 1/16 in (1.6 mm) are generally used to connect the various components of the chromatograph. For the inner diameter there is a choice of 0.18, 0.25, 0.50 and 1.0mm and others (i.e. 0.007, 0.01, 0.02 or 0.04 in). For all connections within the system without any contact to the samples, the 1 mm i.d. capillaries are preferred. They will (usually) not clog and have the lowest possible flow resistance. Their internal volume is 0.8 ml m. For all critical connections with regard to extra-column volumes, i.e. between injector and column as well as between column and detector, it is necessary to use 0.18 or 0.25 mm i.d. capillaries.The extra-column volume is by far lowest with the 0.18 mm type (23 pi m ) but special precautions such as careful filtration of mobile phase and samples are necessary to avoid clogging. The 0.25 mm capillaries (50 pi m ) are less critical but they cannot be recommended for the use with columns which generate very narrow peaks. 

In Figure 22.24, resistance plots for p/ q versus are shown for a series of runs involving Kentucky No. 9 liquefied coal filter in the U.S. Filter Co. vertical leaf filter. They follow the general trends of filtrations subject to both medium and cake clogging as illustrated in Figure 22.23. 

Filtration is a special example of flow through porous media, which was discussed in Chap, 7 for cases in which the resistances to flow are constant. In filtration the flow resistances increase with time as the filter medium becomes clogged or a filter cake builds up, and the equations given in Chap. 7 must be modified to allow for this. The chief quantities of interest are the flow rate through the filter and the pressure drop across the unit. As time passes during filtration, either the flow rate diminishes or the pressure drop rises, In what is called constant-pressure filtration the pressure drop is held constant and the flow rate allowed to fall with time less commonly, the pressure drop is progressively increased to give what is called constant-rate filtration. 

Filter aids may be applied to the filtration operation in two ways. The first way is to precoat the filter medium using a precoat filter aid. The precoat is to behave as the actual filter medium. The function of such an application is to prevent the filter medium from clogging or fouling as well as to facilitate the removal of the formed cake at the end of filtration. The second way is to pretreat the suspension using a filter aid powder with a coarser size distribution prior to the filtration process. Such material is called body aid or admix. The functions of body aid are to increase the porosity of filter cake and to decrease its compressibility, resulting in a decrease in the cake resistance and in turn an increase in the filtration rate. 

The cross-flow filtration device (Figure 14.6(b)) aims to avoid the deposition of particles on the upstream side of the membrane and the associated resistance to filtration. Parallel to the surface of the filter, a flow is maintained, whose velocity is much higher than the velocity of the permeate flow induced by the pressure difference through the filter. The fluid and particles essentially just travel along the surface of the filter, and a small fraction of the fluid flow rate passes through the membrane. The friction of the tangential flow on the filter wall is used to re-entrain the particles that are deposited on the filter, in order to prevent the formation of a cake which would diminish the filtration flow rate. Cross-flow filtration reduces the phenomenon of membrane clogging. 

Since selective formation of a specified pore size has been achieved at a considerable level in the present membrane preparation technology, one important key to further extend spread of the membrane filtration in the fumre is economic efficiency. The key to increase the economic efficiency will be reduction of clogging and increasing mechanical and chemical strength. In this coimeclion, a membrane superior in thermal resistance is advantageous because it can expand the range of application. In the field of pharmaceutical application, for example, steam sterilization resistance may be required. 

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